Nanosprings breakthrough shrinks size of sensors

Portland, Ore. - Quantum dots, nanowires and other nanoscale structures populate the frontier of semiconductor research, collectively aimed at downsizing chip components to the molecular scale. So far piezoelectric materials have been used to fabricate nanowires and nanobelts (ring-shaped nanowires) for experimental nanoscale lasers, field-effect transistors, gas sensors, cantilevers and resonators. But none of these have been "single crystal" and therefore only partially exploit the piezoelectric phenomenon, according to one research scientist, who now claims to have fabricated the world's first single-crystal nanosprings that not only outperform predecessors but also promise to enable single-molecule sensors.

"We have fabricated nanosprings for the first time in single-crystal zinc oxide," said Zhong Lin Wang, director of Georgia Tech's Center for Nanoscience and Nanotechnology and a professor in the School of Materials Science and Engineering. "Quantum dots and quantum wires [nanowires] are mostly semiconductors today, but ours [are] so far the only [single-crystal] piezoelectric nanowires." He performed the work with a fellow research scientist at Georgia Tech, Xiang Yang Kong.

Because piezoelectric semiconductors are natural resonators, they don't need all the support circuitry that normal semiconductors need to make them process and emit signals. When stimulated physically, a piezoelectric material will naturally oscillate at a known frequency. Therefore, if its surface is treated to attract, for example, a protein from a cancer cell, Wang said, then even a single molecule of that protein could be detected with one of his nanosprings.

"We just developed our first application, called the positive resonance technique for detecting biomolecules with nanosprings," Wang said. "If you have a single molecule on the surface [of a nanospring], you can detect a change in its resonant frequency, and by determining the frequency, you can tell what molecule you have."

Wang is now working with multidisciplinary specialists to devise a micron-sized "pill" that disperses millions of such nanosprings all through the entire body, and radios through the skin if cancer cells are detected.

"We want to detect cancer cells," said Wang. "We see the potential for individual, real-time, early detection of cancer, where we transmit a signal wirelessly for about 24 hours. As the nanosprings work their way through your body, they detect any cancer there and send a radio frequency signal out [through the skin]," said Wang.

Wang promises a prototype of his micron-sized "pill" by year's end. Besides biomedical applications, Wang also claims that his nanosprings are compatible with traditional photolithographic techniques of chip-making and that, for traditional sensors and actuators, they can be grown on-chip wherever they are needed as highly sensitive transducers.

"We can control growth with lithography to produce patterns at only the places you want them to grow," said Wang.

History lesson

Piezoelectric materials, discovered by inventor brothers Pierre and Jacques Curie, were first shown to generate electricity from stress in 1880, but the converse effect-stress created from applied electricity (as in today's flat piezoelectric speakers)-was quickly discovered, too (1881). The first real-world application of the material was by French military scientists in 1917, as an ultrasonic submarine detector that was eventually dubbed "sonar."

Since then, piezoelectric materials have been mined to stabilize oscillators (so-called "quartz" frequency references), as sensors of the elastic and viscous properties of liquids and gases, and in acoustic holography that can detect microscopic flaws in structural metals.

Eventually, researchers no longer had to depend on mining piezoelectric materials, learning instead to synthesize them in the lab to produce piezoelectric ceramics for phonograph cartridges, microphones, accelerometers, ultrasonic transducers, bender-element actuators and signal processors such as high-frequency surface acoustic wave filters.

Today, piezoelectric materials are being integrated onto chips as solenoid replacements and are envisioned as the electrostatic "muscles" for future microelectromechanical systems (MEMS), as well as for even smaller nanoscale devices, such as Wang's "nanopill" containing millions of nanosprings, each one of which can independently transmit the results of medical-diagnostic tests wirelessly through the skin.

Wang and Kong fabricated their nanosprings using a solid-vapor process by evaporating high-purity zinc oxide powder in vacuum at 1,350 degrees C, at which time an argon gas flow is introduced into the furnace. The ring shapes began growing on a cooler 400- to 500 degrees C alumina substrate, with the thinnest of them forming spiraled nanosprings.

In practice, Wang's nanosprings measure just 10 to 60 nanometers wide and 5 to 20 nanometers thick, with their coils measuring 500 to 800 nanometers in diameter and up to several hundreds of microns in length. When the material is made, only the thinnest ones naturally spiral into springs. The thicker ones either form rings or just naturally do not spiral at all, because the electrostatic force causing spiraling cannot overcome the structural inelasticity of the thicker material. But when they are thin enough, the zinc oxide naturally spirals, Wang said, because a ring of zinc-oxide molecules cancels out the effect of zinc oxide's electric field-or in physics parlance its "moment"-defined as the tendency to cause rotation about an axis.

Zinc oxide (ZnO) acts as a piezoelectric semiconductor because of asymmetries in its crystalline structure. In particular, with single-crystal ZnO, the oxygen end of its molecule always stays attached to the substrate, while the zinc end always sticks up perpendicular to the surface (called the "c-plane"). Since the zinc ion on the top of the molecule is positively charged, while the bottom oxygen ion is negatively charged, the result is electrostatic polarization all along its surface, causing a perpendicular dipole moment there.

Many asymmetries affect moment, but two oppositely charged surfaces is a classic Physics 101 case. The electric field created by the dipole creates the moment, but when neutralized by spiraling, the material achieves a lower energy state under the laws of thermodynamics (every system naturally seeks its lowest energy state).

"They want to reduce their electrostatic interactions, so they bend into a circle because circles neutralize the dipole moment. Let me give you a simple example. Say you have a flat capacitor, charged positive on the top plate and negative on the bottom plate so it has a dipole moment across the plates. But if you bend the plates into a circle, so that the positive charge is a circle in the center, then the circle of negative charge also has the same center-so the dipole moment vanishes," said Wang.

In the real world, vanishing dipole moments only occur at the nanoscale. "Big" objects (larger than 100 microns or so) do not behave as ideally because of their structural inelasticity, but at the nanoscale objects tend to behave the way an ideal model predicts.

"Vanishing moments only happen when the material is very thin, because it is the elastic energy that matters here. If it's very thin, they can do it-it's like you can bend a piece of paper into a circle, but you can't bend a book, because it takes too much energy," said Wang.

"The spiraling of this nanospring is a charge-induced phenomenon and because it is a piezoelectric semiconductor, very small deformations produce a voltage drop across both sides-it's a very sensitive transducer," he said. "Using individual nanosprings, we can make really nanosized biosensors. We could measure really small changes in pressure in the biofluids of the brain . . . and do wireless detection."